Generally, an enamel wire, which is an insulated wire, is wound in a coiled form in the inside of an electric apparatus, and functions to interconvert electrical energy and mechanical energy by means of a conversion process of magnetic energy. Such an enamel wire is generally composed of a conducting wire such as copper, etc. and an insulated coating layer surrounding the conducting wire, and it is manufactured by applying pigment, generally composed of organic solvent and polymeric resin, to a surface of a conductor, drying and cross-linking it at a high temperature of more than 400° C. to form a thin insulated coating layer. Here, the coating pigment is present in the state that the polymeric resin components are dissolved and dispersed in the organic solvent.
Such an enamel wire has been recently used in various fields such as heavy electric apparatuses, automotive parts, household appliances, medical appliances, and core materials in the aerospace industries, etc.
Recently, there has been required a small and light motor with high performance as the electric and electronic apparatuses increasingly tend toward their miniaturizaion and lightweightness. Accordingly, the winding number of the enamel wire wound around a motor core should be necessarily increased with trends of such miniaturizaion and lightweightness. Also, as the winding number of the enamel wire becomes increased and a size of the motor becomes smaller, an unreasonable process has been carried out, for example to forcibly fit the enamel wire into a core slot. As a result, there often appears a problem that the coating layer of the enamel wire is easily damaged.
Also, as a winding speed is further increased in order to improve productivity using winding machines, tensions and impacts inflicted on the enamel wire are also increased upon its winding, resulting in an increased damage of the insulated coating layer.
Such a damage of the insulated coating layer has a problem that it is a major cause of the poor inherent characteristics of the winding and also the deteriorated reliability of the motor.
In order to solve the problems, there have been attempts to give a lubricating ability to the insulated coating, mainly by adding an organic or inorganic lubricant to a coating pigment, or by directly applying a lubricant oil, etc. onto the enamel wire. However, the aforementioned problems were not fundamentally solved even in such a method.
As an alternative, mechanical strength of the insulated coating of the enamel wire was further increased so as to solve the problems, but simple increase of the mechanical strength allows the coating to be more rigid and less flexible. Accordingly, there are problems that the coating layer is easily cracked and peeled off and properties of the enamel wire are deteriorated when the enamel wire is bent.
In particular, a polyamideimide-based resin, widely used as the coating material of the enamel wire, has excellent physical properties, but it has a disadvantage of having poor adhesivity to a conductor, resulting in deterioration of flexibility and wear resistance of the enamel wire.
In order to solve the problems, various methods have been suggested to increase adhesivity of the polyamideimide-based resin to a conductor, for example by adding alkoxy-modified resin and benzotriazole to a polyamideimide resin solution (Japanese Laid-open Publication No. H3-37283); trialkylamine to a polyamideimide resin solution (Japanese Laid-open Publication No. H6-111632); or melamine resin to a polyamideimide resin solution (Japanese Laid-open Publication No. H10-334735).
However, these kinds of the compositions have problems that their solubility or compatibility is insufficient, or their storage stability is deteriorated since other additives reacts with unreacted terminal groups of the polyamideimide resin, etc.
Also, an additional process such as an undercoating, in which a separate bonding layer is formed between a conducting wire and another coating layer, was required in the prior art to improve an adhesive force between a coating layer and a conducting wire.